PHP Upgrade in Docker Container

I use NextCloudPi in a Docker Container.
By default, NCP has PHP 7.3, but for NC-Passwords, you need 7.4 or higher.
I tried several Pages to update, but after the Update, Nextcloud completly crashed and the php service is dead.
Is there a manual, how to update to a newer PHP version in Docker ?

System: amd64
Ubuntu 20.04 LTS

Nope, not afaik.
Ncp will most likely not get php7.4 untilll Debian 11.
We have had bad experience once, using the Sury repo in the past, to upgrade php, so asume not going there again.

My Nextcloud Shows me, that with Nextcloud 24 PHP 7.3 will no longer supported.
Is now a plan how to upgrade the image and is there a way for me to update for example the Dockerfile to Update to PHP 7.4 or PHP 8?

You can follow and help development here,
where upgrade to Debian 11 (and php7.4) is being tested by and for ncp users.
Should be in time for NC24 …
Current ncp v1.44.5 is at and supports NC22.2.2

If you cant wait, can try Upgrading PHP within a docker container - Unix & Linux Stack Exchange. There are plenty more search results on the web.

official docker images for NC22 and NC23 already use PHP8